Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Electonic scales question...
Summary: Inaccuracy when scale placed on carpet
	 '
Message-ID: <3861@kitty.UUCP>
Date: 17 Jul 90 23:03:44 GMT
References: <5907@videovax.tv.tek.com> <4145@ash31.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 39

In article <4145@ash31.UUCP>, svoboda@motcid.UUCP (David Svoboda) writes:
> > It seems that since the carpet has a springy pad underneath, the scale will
> > read too low.  This is because the scale will only measure compression of
> > its internal spring, and some of that compression gets used up by carpet.
> 
> Wait a second.  Think about this for a moment.  What EXACTLY do you mean by
> "compression get used up by the carpet"?  We are talking about forces here.
> As long as the bottom of the scale is not accelerating wrt the earth, the
> *force* imparted on the scale by the mass of the person is constant, 
> regardless of what surface it rests upon.
> ... 
> I do not submit a reason for the scale not to work on carpet; it has me
> puzzled.

	I can't remain silent any longer.

	The reason why some scales are inaccurate when used on a floor
with carpet has nothing to do with "compression gets used up by carpet",
as stated by the author of the first article.  The author of the second
article explained a rather basic point of physics to further illustrate
why the explanation of the first author is incorrect, but still did not
shed light on this age old mystery.

	Most household scales are made of rather flimsy metal and/or
plastic.  When such a flimsy scale is placed on a spongy surface,
the scale housing will temporarily deform due to unequal stresses
on the case of the scale.  This temporary deformation and unequal
stress will most likely cause an erroneous weight reading due to
spurious stress and/or deformation imparted upon the spring mechanism
and/or LVDT or load cell in the case of an electronic scale.

	Take a good look at a household scale sometime, place it on
a carpet, and shift your weight around while watching a changing
reading.  Try the same thing on a hard, level floor.  The reason
for inaccurate readings on carpet should then be obvious.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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